Investigation of the Response of Binary Multileaf Collimator Compensation to Target Setup Errors in the Radixact Synchrony System: A Phantom Study

The Radixact Synchrony system incorporates real-time target tracking into helical tomotherapy, dynamically correcting motion via jaw tracking and binary multileaf collimator (MLC) offset adjustments. Jaw tracking offers adjustments for longitudinal motion (IEC-Y), whereas MLC-based compensation for transverse motion (IEC-X and IEC-Z) is constrained by the 6.25-mm discrete leaf width, potentially affecting dosimetric accuracy. Furthermore, to formulate effective therapeutic approaches in motion-adaptive radiation therapy, understanding the MLC compensation threshold is crucial. This study aimed to assess the robustness of setup error correction and estimate the binary MLC compensation threshold in the Radixact Synchrony system through a phantom-based dosimetric analysis. Experiments were conducted using the Accuray Precision treatment planning system (version 3.3.1, Accuray, Sunnyvale, CA, USA) and the Radixact X9 (version 3.0.3, Accuray, Sunnyvale, CA, USA). A stereotactic body radiation therapy plan for lung cancer was delivered to a moving phantom using Radixact Synchrony. The phantom motion was simulated using the Delta4 Phantom+ and the Hexamotion stage. To evaluate the impact of setup errors on dosimetric accuracy, measurements were performed under longitudinal and transverse setup error conditions. Dosimetric accuracy was evaluated on the basis of the gamma pass ratio (GPR) and dose profile analysis, and the binary MLC compensation threshold was estimated. Along the longitudinal direction, jaw tracking effectively preserved dosimetric accuracy despite setup errors. Along the transverse directions, the GPR decreased as setup errors increased but improved when MLC compensation became active. Dose profile analysis revealed that MLC compensation was initiated at 4.3 mm. This study evaluated the impact of MLC compensation on dosimetric accuracy and estimated its threshold to be approximately 4.3 mm in the Radixact Synchrony system. Jaw tracking effectively corrects for longitudinal motion, whereas MLC-based compensation in the transverse direction is constrained by its discrete resolution and threshold activation. These findings offer valuable insights for refining treatment margins and enhancing motion-adaptive radiation therapy strategies. To enhance the accuracy and efficacy of Synchrony-based motion correction, studies focusing on clinical validation and parameter optimization are warranted.